QGraphicsDropShadowEffect forces the host widget through an off-screen
software Gaussian blur on every repaint. On the floating sidebar (HomeView,
blur 25) and the player right sidebar (PlayerView, blur 35), the blur was
re-rasterized on every frame of the 350ms slide animation, the main source
of UI jank.
Bake the blur once into a 9-patch PNG (cached to CacheLocation/shadows via
QGraphicsBlurEffect) and render it at runtime as a cheap 9-slice stretch with
zero per-frame blur cost:
- utils/shadowpixmap: off-screen bake + on-disk cache, dpr-aware.
- components/shadowoverlay: mouse-transparent sibling widget, lazy-baked,
paints the 9-patch; optional body cutout for semi-transparent targets so
the shadow does not bleed through the body.
- homeview/playerview: drop the effect; the overlay tracks the sidebar
geometry via the existing eventFilter (Move/Resize/Show/Hide), so every
setGeometry/move site and the slide animation drive it for free.
